Backlight systems, which illuminate a liquid crystal layer from behind, are in widespread use as transmissive liquid crystal display device, and the transmissive liquid crystal display device is equipped with an edge light type (side light type) backlight unit, an immediate beneath type backlight unit or the like on the inferior face side of the liquid crystal layer. The edge light type backlight unit 40 is generally equipped with a lamp 41 as a light source, an optical waveguide plate 42 in the form of a rectangular plate arranged with its edge aligned with the lamp 41, and a plurality of optical sheets 43 superposed on the front face side of the optical waveguide plate 42, as shown in FIG. 7. LEDs (light emitting diodes) and cold-cathode tubes and the like are used as the lamp 41 as the light source, and the LEDs are currently gaining widespread use from the viewpoint of the size reduction and energy saving and the like. The optical sheet 43 has optical functions such as diffusion and refraction of the transmitted rays of light and the like, and as the optical sheet 43, (1) a light diffusion sheet 44 which is disposed on the front face side of the optical waveguide plate 42 and primarily has the light diffusion function, and (2) a prism sheet 45 which is disposed on the front face side of the light diffusion sheet 44 and has the refraction function toward a normal direction side, and the like are utilized.
Although not shown in FIG. 7, there are also some backlight units in which much more optical sheets 43 including the light diffusion sheet and the prism sheet are disposed, considering the optical waveguide characteristics of the optical waveguide plate 42 and the optical functions of the optical sheet 43 and the like, as described above.
The functions of the backlight unit 40 will be explained; first, the rays of light entering the optical waveguide plate 42 from the lamp 41 are reflected on reflection dots or a reflection sheet (not shown) on the back face of the optical waveguide plate 42 as well as on each lateral face, and are output from the front face of the optical waveguide plate 42. The rays of light output from the optical waveguide plate 42 enter a light diffusion sheet 44, and are diffused and output from its front face. The rays of light output from the front face of the light diffusion sheet 44 enter a prism sheet 45, are refracted by a plurality of protruding prism portions formed on the front face toward a normal direction and output, and illuminate the entire liquid crystal layer not shown) located further above the prism sheet 45.
The light diffusion sheet 44 substantially uniformly disperses the transmitted rays of light, and is used for the purpose of uniformalization of luminance, luminance heightening in the front direction and the like due to its light diffusibility. As the light diffusion sheet 44, there is commonly used a light diffusion sheet (see, for example, Japanese Unexamined Patent Application, Publication No. 2004-198743) that includes a substrate layer 46 made from a synthetic resin, and an optical layer 47 laminated on the front face of the substrate layer 46, in which the optical layer 47 is made form a blended material of spherical resin beads 50 in a binder 49, as shown in FIG. 7(b).
However, in order to increase light diffusibility for the purpose of the uniformalization of luminance and the like, a blended amount of the resin beads 50 in the resin-beads-coating type light diffusion sheet 44 is needed to be increased; however, such an increase in the blended amount of the resin beads 50 leads to reduction of face luminance. In other words, in the beads-coating type light diffusion sheet 44, there is a trade-off relationship between the light diffusibility and the face luminance.